In a manufacturing process of a semiconductor device or a flat panel display (FPD), a plasma processing apparatus for performing etching, deposition, oxidation, sputtering or the like by using a plasma is widely used. In the plasma processing apparatus, in order to use a RF power for plasma generation, a RF power of a predetermined frequency (generally, 13.56 MHz or above) is supplied from a RF power supply unit to a RF electrode (or an antenna) provided inside or outside a chamber. Further, in order to freely control energy of ions incident to a target substrate from a plasma, a RF power of a predetermined frequency (generally, 13.56 MHz or less) is supplied from the RF power supply unit to a RF electrode of a mounting table for mounting thereon the substrate.
The RF power supply unit includes a RF power supply for outputting a RF power and a matching unit for matching an impedance of the RF power side and an impedance of a load side (electrode, plasma and chamber). The RF power supply and a transmission cable are designed to have an output resistance of about 50Ω, and the impedance in the matching unit is set or controlled such that the impedance of the load side including the matching circuit becomes about 50Ω, i.e., such that the power of the reflected wave becomes minimum.
In general, the matching unit used in the plasma processing apparatus includes a plurality of variable reactance elements, and is configured as an automatic matching unit capable of variably controlling the load impedance by selecting impedance positions or reactances of the variable reactance elements by a stepping motor or the like.
If the impedance of the plasma load changes due to a pressure change or the like during the plasma processing, the automatic matching unit automatically corrects the load impedance to a matching point (50Ω) by variably controlling the reactances of the variable reactance elements. In order to perform the automatic matching operation, the automatic matching unit is provided with a circuit for measuring a load impedance, a controller that variably controls a reactance of each variable reactance element by a stepping motor to match the measured value of the load impedance to the matching point (50Ω), and the like.
In general, the automatic matching unit includes two variable capacitors serving as the variable reactance elements in the matching circuit, the variable capacitors being respectively connected in parallel and in series to the load with respect to the RF power supply. Here, the electrostatic capacitance of the first variable capacitor connected to the load in parallel operates to variably control mainly the absolute value of the load impedance. Meanwhile, the electrostatic capacitance of the second variable capacitor connected to the load in series operates to variably control mainly the phase of the load impedance (phase difference between RF voltage and RF current).
A conventional typical automatic matching unit varies the electrostatic capacitance (capacitance position) of the first variable capacitor in a stepwise manner such that a measured absolute value and a measured phase of a load impedance obtained by an impedance measuring circuit become close to matching point values, i.e., a reference absolute value and a reference phase, respectively, and also varies the electrostatic capacitance (capacitance position) of the second variable capacitor such that the phase error becomes close to zero (e.g., Japanese Patent Application Publication No. H10-209789).
In the plasma processing apparatus, the impedance of the plasma load changes dynamically and indefinitely due to a pressure change in the chamber or the like. Therefore, the automatic matching unit needs to perform an automatic matching operation capable of responding to changes in the load impedance rapidly and accurately.
However, in the conventional automatic matching unit, a first and a second feedback control system for variably controlling the electrostatic capacitances (capacitance positions) of the first and the second variable capacitor in accordance with the measured value of the load impedance obtained by the impedance measuring circuit are configured to operate at a constant proportional gain (proportional sensitivity).
However, the load impedance varies near the matching point such that the variation of the load impedance per one step in the second feedback control system, particularly the variation of the phase increases remarkably, which may lead to hunting. Therefore, the proportional gain of the second feedback control system is adjusted in advance so as to be relatively smaller than that of the first feedback control system. However, this causes unnecessary speed decrease in a matching operation and cannot be a radical solution.